1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display with less defects during manufacturing processes.
2. Description of the Related Art
FIG. 3 is a cross sectional view of a thin film transistor (TFT) type liquid crystal display manufactured by a conventional method.
Referring to FIG. 3, a twist nematic type liquid crystal cell is constructed Of liquid crystal molecules 30 having longitudinal axes orientated in parallel to the surfaces of two transparent glass substrates, the longitudinal axes being twisted by 90 degrees in the range between one substrate and the other substrate. The liquid crystal layer 31 is retained between the two transparent glass substrates 32 and 33 facing each other and spaced by a predetermined distance by a spacer (not shown).
Formed on the lower glass substrate 32 shown in FIG. 3 are TFTs 34 for applying an electric field to a pixel in response to a gate signal, electrodes (not shown) of source, drain, and gate of TFTs 34, matrix lines (not shown) constituted by gate (scan) lines and signal lines connected to TFTs 34, and pixel electrodes 35 connected to TFTs 34. An orientation film 36 is formed on these elements, the film subjected to the orientation process (rubbing process and the like).
A common electrode 37 is formed on the upper glass substrate 33 shown in FIG. 3. Another orientation film subjected to the orientation process is formed at the interface between the common electrode 37 and liquid crystal layer 31. The upper and lower glass substrates 33 and 32 are disposed with their orientations being perpendicular to each other.
In some cases, there are formed color filter layers (not shown) and a light shielding film (not shown) called a black mask for improving the contrast of an image by preventing light from being transmitted through the area other than the pixel area.
The liquid crystal layer 31 shown in FIG. 3 is a twist nematic type liquid crystal layer. Because the orientation films 36 and 38 were subjected to the rubbing process to make their orientations perpendicular to each other by 90 degrees, when an electric field is not applied between these electrodes as indicated at (A) of FIG. 3, the longitudinal axes of the liquid crystal molecules 30 are orientated generally perpendicular to the optical axis 0X, the longitudinal axes being twisted by 90 degrees in the range between the pixel electrode 35 and common electrode 37. Under this condition indicated at (A) in FIG. 3, when a linearly polarized light having a vertical polarization relative to the surface of the drawing sheet of FIG. 3 becomes incident to the liquid crystal layer 31 from under the glass substrate 32 along the optical axis 0X, the incident light is twisted by 90 degrees and is outputted from the glass substrate 33. This outputted light transmits through a polarizer (not shown) having a polarization axis perpendicular to that at the light incident side, to provide a bright state.
When an electric field is applied between the pixel electrode 35 and common electrode 37 sandwiching the liquid crystal layer 31, the liquid crystal molecules 30 are orientated all in the optical axis direction as indicated at (B) of FIG. 3. Therefore, the incident linearly polarized light passes through the liquid crystal layer 31 without being twisted, and intercepted by the polarizer having the polarization axis perpendicular to that on the light incident side, to provide a dark state.
A conventional method of manufacturing such twist nematic liquid crystal displays is as follows.
First, TFTs 34, matrix lines (not shown) constituted by signal lines and scan lines, and pixel electrodes 35 are formed on the glass substrate 32 and interconnected to make a TFT substrate.
Next, a common electrode 37 is formed on the other glass substrate 33 to make a common electrode substrate. Orientation films 36 and 38 fare formed on the TFT substrate and common electrode substrate and are subjected to the rubbing process.
The substrates are mounted on a gap regulator (not shown) interposed therebetween, while positioning the substrates so as to obtain an orientation difference of 90 degrees between the orientation films 36 and 38. Nematic liquid crystal is filled in between the substrates via an inlet port which is thereafter sealed to obtain a finished liquid crystal display.
The above-described manufacturing method is basically applicable also when using metal-insulator-metal (MIM) diodes instead of TFTs for the control of an active matrix display.
With the above-described manufacturing method, static electricity may generate during the rubbing process for orientation films. This static electricity may cause short-circuits between TFTs or MIM diodes, breakage of lines, destruction of TFTs, or characteristic changes, resulting in possible dot detects or line defects.
Very thin insulating films in the order of 200 to 600 nm are used to electrically insulate between electrodes of TFT or MIM elements and between matrix lines including signal and gate lines. The dielectric breakdown of such thin films may be caused by static electricity. Because amorphous silicon or polysilicon semiconductors are used as TFT material, the characteristics of transistors such as threshold values may change when a high electric field concentrates on electrodes.